The present invention relates to a process for exterior coating a lamp, i.e. for providing a coating on an exterior or outside portion of it. The invention also relates to an apparatus for exterior coating of lamps and, more particularly, to an apparatus for coating an exterior portion of a lamp including a reactor chamber in which the lamp is arranged and a device for conducting a coating gas into and out of the reactor chamber.
The exterior coating of glass bodies is of interest especially for incandescent lamps, because the spectrum of the radiation emitted by the lamp may be influenced because of it in order for example to obtain a certain color effect or in order to reduce the energy consumption of the lamp.
Incandescent lamps radiate a large portion of their input energy in the form of thermal radiation and not as light. In order to utilize the undesirable thermal radiation it is known to reflect the thermal radiation back to the filament by a infrared reflective coating applied to the outer surface of the lamp and thus to reduce the energy consumption required for maintaining its operating temperature. This is, for example, described in IEE Proceedings-A, Vol. 10, No. 6, November 1993, p. 418.
The proportion of the thermal radiation produced by an uncoated halogen lamp amounts to at least 80%. If a halogen lamp is provided with a suitable coating, the energy consumption may be reduced theoretically to about 32% of the energy consumption of an uncoated lamp. According to the current state of the art the proportion of the thermal radiation is always still at 70%. There is thus a great interest in further development of coatings for incandescent lamps.
The coating generally comprises a multi-layer interference system of high-refractive and low-refractive layers, advantageously low-refractive SiO.sub.2 and high-refractive TiO.sub.2 or Ta.sub.2 O.sub.5 layers.
The layers were applied according to different methods on the incandescent lamp. An expensive dipping method is disclosed in European Patent Application EP 0 305 135 and a CVD method is disclosed in European Patent Application 0 369 253. Ta.sub.2 O.sub.5 or SiO.sub.2 layers are formed on the lamp by means of a direct current-cathode sputtering in a PVD process according to European Patent application EP 0 409 451, in which an oxidation is performed after deposition of a thin layer of Ta or Si. These steps are performed one after the other until the resulting Ta or Si oxide layer has reached the required thickness.
These known methods have the disadvantage of a long duration processing time lasting several hours for making the layer packet with the risk of a failure of plant components proportional to the processing time. Only layers with comparatively low refraction number can be applied by means of a dipping method and the CVD method, so that the total number of layers of packets must be increased in order to obtain the required IR reflection. The mechanical expense is very large in the PVD method according to European Patent Application EP 0 409 451, and also the susceptibility for trouble is increased.
German Patent Application DE 3632748 C2 describes a method of coating, especially of coating the interior of a hollow body with a polymeric coating by plasma polymerization. The hollow body is inserted in a microwave chamber, in which microwaves are supplied from the exterior to several locations, so that a homogeneous electric field exists in the chamber. Either monomers provided for the coating or a mixture of monomers and a plasma carrier are conducted into the interior of the hollow body by means of a nozzle shielded from the microwaves, so that a plasma is ignited and the plasma polymerization can be performed. A hollow body having an outer layer of metal is mentioned in this pre-publication, as in "Surface and Coatings Technology", 80, pp. 200-202 (1996), where a PICVD method for making a layers system sufficiently uniform for application as a diffusion blocking layer on the inner surface of a plastic container is described. Although microwave-plasma-CVD methods are suitable basically for coating of glass bodies, these methods were not used up to now for coating the exterior of glass bodies, which have metal components, such as incandescent lamps. The reason is possibly the fear that the conductive regions such as the lamp filament and the connector pins would perturb the original field distribution by reflections.
Another reason is probably that the preferred lamps to be coated are halogen lamps whose power limit is typically about 60 watts, microwave CVD plasmas however operate in the power range of 100 watt to several kilowatts. If the power from the microwave is coupled into the metallic components of the lamp, melting can destroy these components.
Another reason is that a plasma can be excited in the interior of the halogen lamp. Plasmas can be produced with microwaves in a large pressure range from 10.sup.-3 Pa to atmospheric pressure. The gas in the interior of the halogen lamp is at atmospheric pressure and can be excited to a Br- or I-containing plasma with microwaves in contrast to other excitation frequencies, which destroys the lamp filament and can dirty the interior surface of the lamp.